Genetic modification or transformation is the technique wherein one or a few gene(s) are added to a commercially interesting genotype or clone. In principle a successful transformation system requires an efficient system where new plants are formed from specific plant parts (stem, leaf, node and so on) or from specific tissue (somatic embryos, embryogenic callus, friable embryogenic callus) induced on specific plant parts or from protoplasts (single cells without cell wall) derived of these parts or from the specific tissue, a system to transfer DNA molecules to the plant's parts or protoplasts and a system to select tissue and plants which contain and express the introduced gene(s). In principle protoplasts are the most ideal system for DNA delivery. They can be cultured as single cells that produce multicellular colonies from which plants develop. Plants derived from protoplasts are generally clonal in origin. This provides a useful tool for any transformation system, because it will eliminate chimerism in transgenic plants.
Cassava is very recalcitrant for plant regeneration of protoplasts. There is only one report of shoot regeneration from protoplasts of cassava (Shahin and Shephard, 1980). They used well expanded leaves for the isolation of protoplasts. Despite considerable efforts, plant regeneration from protoplasts (isolated from leaves, sterns, and roots) has never been repeated since then (Anonymous, 1985; Nzoghe, 1991; Anthony et al., 1995, Sofiari, 1996). A logical approach was to use tissues which contain embryogenic cells. Such cells are found in the apical meristems, young leaves or somatic embryos cultured on auxin supplemented media (Stamp and Henshaw, 1987a; Raemakers et al., 1993a). However, protoplasts isolated from these tissues gave in the best case green callus and adventitious roots (Sofiari, 1996). Recently, a new type of somatic embryogenesis was developed. In this in vitro system the embryos do not develop beyond the (pre-)globular stage and the embryogenic callus is highly friable (Taylor et al., 1995). Transfer of this friable embryogenic callus (FEC) to liquid medium resulted in a suspension-like culture. In leek (Buitenveld and Creemers, 1994), petunia (Power et al., 1979), rice (Kyozuka et al., 1988), sugarcane (Chen, et al., 1988), and wheat (Chang et al., 1991) such cultures were an excellent source for protoplast regeneration. We have now found that in cassava FEC is the only tissue from which protoplasts can be isolated which are able to regenerate into plants sofar. We have further found that FEC can be used to regenerate cassava plants which tubers contain starch containing substantially no amylose.